This invention relates to devices that generate heat by non-electrical means for the purpose of warming. In particular, the invention relates to a warming pack which generates heat that is used for warming objects such as tissue. In such a device, heat may be generated by an exothermic chemical reaction that takes place within the warming pack. Warming packs that operate based on exothermic chemical reactions are capable of storing energy at ambient temperatures for extended periods of time and then being brought into use very quickly.
A significant problem attending the use of a warming pack is spatial non-uniformity of temperature produced by the warming pack. The material comprising the reactants that drive the heat-generating chemical reaction tends to move or shift during handling of the device. The material is usually in a particulate, powdered or granular form, making it more difficult to contain as the size of a warming pack increases. As the material shifts, localized concentrations can occur within a warming pack, producing variations in temperature across a surface of a warming pack through which heat is transferred for warming. Such concentrations can produce hot and cold spots in a warming pack during operation.
A second problem with warming pack operation is temporal non-uniformity of temperature produced by the warming pack. Chemical warming packs are designed to operate within a temperature range that is safe and effective for some intended use. However, such devices utilize a constant heat generation process, not a constant temperature process. For example, chemical warmers that generate heat via the reduction-oxidation of iron with oxygen can achieve a constant rate of heat generation by metering the amount of oxygen available to react with the iron. For such a pack the quantity of heat that can be generated depends on the amount of iron available. The duration of heat generation depends on the amount of iron available and the rate at which oxygen is made available to react with the iron. Both of these factors are predetermined by the design of the pack. The final steady state operating temperature, however, depends on the heat generation rate and ambient conditions, including temperature. The operating temperature of such chemical warming packs, therefore, will vary with the deviation of ambient conditions from those assumed in the design of the pack.